Method for producing foamed slag on high-chromium melts in an electric furnace

ABSTRACT

The invention relates to a method for producing foamed slag on high-chromium steel melts in an electric furnace, whereby a mixture consisting of a metal oxide and carbon is introduced into the furnace, the metal oxide in the slag is reduced by the carbon, and the gases created in the slag form bubbles which thus foam up the slag. In order to be able to control the gas formation and thus the foaming process, the mixture consisting of a metal oxide and carbon and optionally an iron carrier is introduced as preforms, such as pellets, which are compressed and/or provided with a binding agent. The gas formation can be controlled in terms of location, type and time, by adjusting the characteristics of the pellets, especially the density and/or compression characteristics thereof.

The invention concerns a method for producing foamed slag onhigh-chromium steel melts in an electric arc furnace, wherein a mixtureof a metal oxide and carbon is introduced into the furnace, the metaloxide is reduced by the carbon in the slag, and the resulting gasesbring about the foaming of the slag by bubble formation.

In the operation of electric arc furnaces, the charge, i.e., mainlyscrap, and alloys are melted by the electric arcs of the electrodes,which extend downward into the furnace shell. The slag fulfils not onlyits primary function, i.e., the removal of undesirable components fromthe melt, but also a protective function in the foamed state. In thisstate, the slag encloses the space between the ends of the electrodesand the surface of the metal and protects the refractory lining of thefurnace from radiant energy of the electric arc. Due to the low thermalconductivity of the foamed slag, the radiation of the electric arctowards the wall of the arc furnace is greatly reduced, and thus theenergy input into the metal melt is improved.

In the case of nonstainless steels or steels with a low chromiumcontent, the foamed slag is produced by simultaneous injection of carbonand oxygen onto the slag and into the steel bath, respectively. The gasthat evolves during the reactions that occur:2[C]+{O₂}* 2{CO}2{CO}+{O₂}* 2{CO₂}causes foaming of the slag. In addition, the carbon reduces the ferrousoxide to iron and carbon monoxide according to the equation(FeO)+[C]*[Fe]+{CO}.

The foamed slag encases the electrodes and is present as a protectivelayer between the electric arcs and the furnace wall.

In the case of high-chromium melts, the injected carbon reacts basicallyas a reducing element of the chromium oxide. The reactions specifiedabove have little importance in the metal bath. Besides, the content ofiron in the slag is too low to guarantee satisfactory foaming of theslag. All together, in the case of high-chromium melts, it is difficult,due to the differences that have been mentioned, to produce a foamingslag in the superheating phase.

For this purpose, EP 0 829 545 B1, which concerns a method for producinga foamed slag on molten stainless steel in an electric arc furnace,proposes that a powder, which consists of a metal oxide, either zincoxide or lead oxide, and carbon, be introduced into the slag. The oxidecontained in the powder is reduced by reaction with the carbon. Bubblesconsisting mainly of carbon monoxide are formed in the slag and causethe slag to foam. The powder is introduced into the slag with the aid ofan injection medium, for example, nitrogen.

Thus, in accordance with the prior art, the reactive mixture isintroduced into the slag or melt as a powder. Due to the relativelylarge surface area associated with the powdered form, brief, violentreactions occur. Moreover, the reaction is locally limited in thevicinity of the injection device and here especially at the tip of theinjection lance in the molten bath.

Proceeding from this type of prior art, the objective of the inventionis to develop a method for producing foamed slag on molten high-chromiumsteels in an electric arc furnace, in which the processes that initiatethe foaming reaction occur in a controlled way.

This objective is achieved by a method with the features of claim 1.Advantageous refinements of the invention are described in the dependentclaims.

In accordance with the invention, the furnace is charged with a mixtureof a metal oxide and carbon, not as a powder but rather as compressedpreforms and/or preforms provided with a binder. In addition to thepreferred pelletized form, it is possible to use other forms, forexample, the briquet form. The systematic adjustment of the propertiesof the preforms, hereinafter referred to in terms of their embodiment aspellets, makes it possible, in contrast to use in powdered form, tocontrol the evolution of gas with respect to location, type, andtime—especially the starting point with respect to time, the rate, theintensity of the reaction, and/or the duration of the reaction.

In particular, the density properties of the pellets are adjusted by thecompression pressure and/or the type and amount of an admixed ironcarrier, for example, ferronickel, and a binder. In this regard, inaccordance with a preferred variant, the density of the compressedpreforms is adjusted in such a way that the pellets float in the slagnear or directly on the metal-slag phase boundary itself. The additionof the iron carrier ensures that the pellets are heavier than the slagbut lighter than the metal melt. The evolution of gas thus occurs in alocally well-defined way, namely, in the slag at the boundary betweenthe metal and slag. In this way, there is no contact between pellets andmetal bath, so that carburization of the melt is prevented. It is alsopossible to adjust the pellet properties in such a way that the pelletscan occupy different positions between the molten bath and the slag.This guarantees that the processes that initiate the foaming occur onlyin the slag, so that the effectiveness is increased.

Furthermore, the pellets should have a density or a degree of compactionthat causes them to disintegrate uniformly and slowly, so that thefoaming reaction occurs uniformly and over a relatively long period oftime. In addition, it is possible to cause the reactions to occur with atime delay by using even higher pressure compaction. This prevents thereaction from occurring too soon and guarantees that the reaction willnot start until the pellets are distributed in the slag.

In addition, the evolution of gas can be systematically adjusted by thesize of the pellets. As a result of the fact that the pellets have arelatively large diameter and thus a smaller specific surface thanpowders, the foaming reaction can be maintained for relatively longperiods of time with uniform gas evolution.

The basic components metal (Me) oxide and carbon are involved in thefollowing reactions:(Me_(x)O_(n))+[C]·x[Me]+{CO}2{CO}+{O₂}2{CO₂}

Waste products of steel production can be used for the mixture forproducing the pellets, such as carbon from consumed electrodes or piecesof waste scale. The use of binders is advisable especially with mixturesof this type.

Aside from the basic components metal oxide and carbon, a flux,especially limestone, is additionally pressed into the proposedpelletized form. The desired CO/CO₂ formation is additionallyintensified by the limestone.

Furthermore, a slag thinner, preferably CaF₂, can be additionallypressed into or bound with the mixture. This counteracts the tendency ofchromium-containing slags to become increasingly viscous with increasingchromium oxide content.

It is also advisable to press a reducing agent, such as silicon and/oraluminum, into some of the pellets, especially together with limestone,to control the chromium oxide content of the slag. These reducing agentsreduce the chromium oxide contained in the slag and thus lower thechromium content of the slag. In addition, the foaming of the slag isimproved.

In contrast to powder, which must be locally injected, the pellets areadded in various parts of the furnace through the furnace roof and/orthe sidewalls of the furnace. This is not possible with powder, becauselarge fractions of the powder would be sucked out by the dust removalsystem of the furnace. It is also advisable to introduce the pelletsinto the slag in a directed way in the vicinity of and directly at thehot spots of the electrodes to allow the foaming process to occurespecially at the electrodes.

Additional details and advantages of the invention are specified in thefollowing description of the drawings.

FIG. 1 shows a schematic representation of the cross section of anelectric arc furnace with charging devices for the slag-foaming pellets.

FIG. 2 shows the furnace in FIG. 1 from above.

The electric arc furnace 1 shown in FIG. 1 comprises a furnace shell 2with a refractory wall 3 and a furnace roof 4. After the furnace hasbeen charged with scrap and alloying components, three electrodes (inthe present case) 5 a-c are lowered into the interior of the furnace.The solid material is melted down by the electric arcs that areproduced. A slag layer 7 is formed and floats on the melt. To initiate afoaming reaction of the slag 7 between the electrodes 5 a-c and therefractory furnace wall 3, slag-foaming material is introduced into theinterior of the furnace as preforms 8, namely, in the form of pellets.The pellets are preferably charged through the furnace roof 4,specifically, through the fifth roof hole 9, and/or the sidewalls 10.Injection-systems with injection lines or gravity feed systems 11 thatextend through the sidewalls 10 of the furnace are provided for thispurpose. Instead of injection lines, it is also possible to useinjection lances.

Alternatively or additionally, a pneumatic conveyance system 12consisting of closed circular pipelines is also suitable for chargingthe pellets. This system has a closed circular pipeline 13 that runsalong the roof 4, as shown in FIG. 2, which at the same time also hasclosed circular pipeline segments 14 that run radially to the roof.Three charging holes 15 a-c (in the illustrated example) are provided inthe closed circular pipelines 13, 14 and the corresponding roof wall.The pellets are introduced into the furnace slag 7 uniformly over thecross section of the furnace by this system 12. In this regard, thecharging holes 15 a-c are arranged in such a way that the pellets reactwith the slag 7 in the vicinity of the hot spots.

The pellets float in the slag 7, where they react to produce the desiredgas evolution and thus foaming reaction in a way that is controlled withrespect to location, time, and type. In particular, the adjustment ofthe density and size of the pellets makes it possible to ensure that thegas evolution process proceeds as uniformly as possible, for arelatively long time and not too violently. A controlled reaction at thesurface of the pellets results in uniform foaming of the slag.

LIST OF REFERENCE NUMBERS

-   1 electric arc furnace-   2 furnace shell-   3 refractory wall-   4 furnace roof-   5 electrodes-   6 melt-   7 slag-   8 preforms (pellets)-   9 fifth roof hole-   10 sidewalls of the furnace-   11 injection line-   12 pneumatic conveyance system-   13 closed circular pipeline-   14 sections of closed circular pipeline-   15 charging holes

1. Method for producing foamed slag (7) on high-chromium steel melts (6)in an electric arc furnace (1), wherein a mixture of a metal oxide andcarbon is introduced into the furnace (1), the metal oxide is reduced bythe carbon in the slag (7), and the resulting gases form bubbles in theslag, which thus cause the slag to foam, wherein the mixture of metaloxide and carbon is introduced into the furnace as compressed preforms(8) and/or preforms (8) provided with a binder.
 2. Method in accordancewith claim 1, wherein the density of the preforms (8) is adjusted insuch a way that they float in the slag (7).
 3. Method in accordance withclaim 1 or claim 1, wherein the density of the preforms (8) is adjustedin such a way that they float in the slag near the phase boundarybetween the melt (6) and the slag (7).
 4. Method in accordance withclaim 2 or claim 2, wherein the density of the preforms (8) is adjustedby the addition of an iron carrier.
 5. Method in accordance with claim1, wherein the density of the preforms (8) is adjusted in such a waythat they disintegrate in the slag (7) uniformly and slowly, and theevolution of gas occurs uniformly and over a relatively long period oftime.
 6. Method in accordance with claim 1, wherein the density of thepreforms (8) is adjusted in such a way that they disintegrate with atime delay.
 7. Method in accordance with claim 1, wherein a flux,preferably limestone, is additionally added to the mixture.
 8. Method inaccordance with claim 1, wherein a slag thinner, preferably CaF₂, isadditionally added to the mixture.
 9. Method in accordance with claim 1,wherein a reducing agent, preferably silicon and/or aluminum, isadditionally added to the mixture.
 10. Method in accordance with claim1, wherein the preforms (8) are introduced through the sidewalls (10)and/or the furnace roof (4) of the electric arc furnace (1).
 11. Methodin accordance with claim 1, wherein the preforms (8) are introduced intothe slag (7) in a directed way in the vicinity of or directly at the hotspots of the electrodes (5 a-c).